Electrostatic electrophotography involves the use of a photosensitive member having a photoconductive insulating layer which is first uniformly electrostatically charged. Electromagnetic radiation, such as light, X-rays or the like, dissipates the charge in areas of the photoconductive insulator to which it is directed causing a latent electrostatic image in the areas where such radiation is not directed. The latent electrostatic image that is produced can be made visible by various development processes such as those in many patents issued to the Xerox Corporation in the field of xerography and those described in U.S. Pat. No. 3,909,258 to Kotz and U.S. Pat. No. 4,121,931 to Nelson.
Extensive use has been made in commercial electrostatic electrophotography of vitreous selenium as a photoconductor, as desribed by Bixby U.S. Pat. No. 2,970,906, since it is capable of holding and retaining an electrostatic charge for relatively long periods of time when not exposed to light and is relatively sensitive to light as compared to other photoconductors. It also possesses sufficient strength and stability to enable it to be reused thousands of times. The effective life of selenium as a photoreceptor member is limited, however, since vitreous selenium is susceptible to deleterious crystal growth. In addition, the spectral response of vitreous selenium, which is limited to the blue or blue-green range of the visible spectrum, restricts its use for some applications in electrostatic electrophotography.
U.S. Pat. No. 2,803,542 to Ullrich and 2,822,300 to Mayer et al, both teach the common concept of improving the property of vitreous selenium by the addition of elemental arsenic in amounts up to about 50% by weight. The addition of arsenic greatly increases the stability of selenium at elevated temperatures and increases spectral response in the yellow-red band of the electromagnetic spectrum when arsenic concentrations greater than 10% by weight are used. Concentrations of arsenic greater than about 10%, however, cause a vitreous selenium photoreceptor to retain a high residual potential with positive charging, and in addition, cause high light fatigue. Very faint, residual negative images in background areas appear after extended repetitive imaging when there is high light fatigue.
Two-layered receptor structures have been designed to overcome some of the above-noted disadvantages. These structures, for example, contain layers of selenium and selenium-tellurium alloys. U.S. Pat. No. 2,803,541 to Paris illustrates one such patent in which improved photosensitivity is attained by using a top layer of vitreous selenium-tellurium over a layer of selenium. The structure provides limited abrasion resistance for automatic copy machine operation and also exhibits high dark discharge. While protective organic and inorganic overcoatings have been used to provide improved abrasion resistance such overcoatings do not function properly through a wide range of environmental conditions. Such overcoatings are known to be humidity sensitive causing image quality problems.
U.S. Pat. No. 3,655,377 to Sechak provides a tri-layer photoreceptor member which overcomes the disadvantages presented by the above-mentioned two layered and the overcoated types of two layered photoreceptor members. Sechak's tri-layer photoreceptor member utilizes a top layer or overcoating of arsenic-selenium alloy for abrasion resistance, temperature stability and improved dark discharge.
It can be seen that the evolution of a photoreceptor member to provide one that is panchromatic responsive, abrasion resistant as well as thermally and humidity stable and not subject to fatiguing effects has resulted in tri-layered photoreceptor structures. This approach, of course, complicates the process of manufacture in that three layers are involved.